That PR/MR icon is quite terrible, I much prefer github's which actually communicates the idea clearly and abstractly and is uniquely recognisable. This other one is just two squares, it could really mean anything.
That is fair, I'd say the same thing about "Pull Request" as a name when compared to "Merge Request" though - it's so counter-intuitive to people new to the process.
It looks like that icon is not part of a public icon library, but the concept of an arrow joining one node to another node in a graph does not belong to github, it just needs an originally drawn icon is all.